The present invention relates to a battery system and, more particularly, to a battery system for a motor vehicle having a cell arrangement including several battery cells connected in series, and a transformer having a first winding, which is connectable in parallel with the cell arrangement and several second windings, each second winding being connectable in parallel with one of the battery cells.
A known battery system of the above-type is described in the document entitled “Active Charge Balancing for Lithium-Ion Batteries”, which was published in the journal ATZ Elektronik February 2008, Year 3, Pages 62-66 and authored by Werner Rößler (the “Rößler reference”).
Battery systems of hybrid vehicles have a plurality of battery cells connected in series. It is known that, for obtaining a service life that is as long as possible, the charging state level of the battery cells should be maintained in a certain range. If a battery is overcharged, i.e. the charging state level rises above a predefined value, or if the battery is excessively discharged, i.e. the charging state level falls below a predefined value, such overcharging or excessive discharging has a very negative impact on the service life of the battery.
In the above-mentioned Rößler reference, a battery system is described in the case of which, by means of a transformer device controlled by an electronic battery management system, an active charge balancing can be carried out between individual battery cells of the battery system that are connected in series. The term “active charge balancing” means that energy is “displaced” between the cells as soon as the voltage of a battery cell deviates too far from the average voltage of the battery cells that are connected in series. A corresponding circuit arrangement is illustrated on page 64 of the Rößler reference. The transformer arrangement, by which the active charge balancing is carried out, has a first winding which is connected in parallel with the cell arrangement formed by the battery cells, which cells themselves are connected in series. Furthermore, several second windings are provided, in which case each such second winding is connected in parallel with the battery cells that are connected in series. One switch respectively constructed as a MOSFET is assigned to the first winding and each second winding. The switches are triggered by an electronic battery management system.
Basically, there are two cases in which charge balancing is to be carried out They are called “bottom balancing”, on the one hand, and “top balancing”, on the other hand.
“Bottom balancing” takes place when the charging state level of one of the battery cells connected in series has fallen below a permissible value. In this case, the “weak battery cell” has to be charged. For this purpose, the switch assigned to the first winding is closed, which has the result that the transformer is magnetically charged from the entire battery block. After the switch assigned to the first winding has been opened again, the magnetic energy stored in the transformer can be displaced in a targeted manner into the “weak battery cell”, in that the switch assigned to this cell is closed.
“Top balancing” takes place when the charging state level of one of the battery cells connected in series exceeds a predefined value. In this case, the switch assigned to the “overcharged battery cell” is closed, which has the result that the transformer is charged by way of the second winding assigned to the overcharged battery cell. Subsequently, this switch is opened and the switch assigned to the first winding is closed. The energy stored in the transformer and supplied by the overcharged battery cell will then be fed by way of the first winding back into the entire cell arrangement formed by the individual battery cells connected in series.
Such a charge balancing can be carried out very effectively, i.e. with a high frequency.
It is an object of the invention to further develop the battery system, which is known from the state of the art and permits an active charge balancing, particularly such that it can be charged from the outside in a simple manner.
This and other objects are achieved by a cell arrangement including several battery cells connected in series, and a transformer having a first winding, which is connectable in parallel with the cell arrangement and several second windings, each second winding being connectable in parallel with one of the battery cells. The transformer includes a third winding which is provided for feeding energy from an external energy source to the battery system.
The starting point of the invention is a battery system having a cell arrangement, which arrangement has several battery cells connected in series, and a transformer which permits an active charge balancing between the individual battery cells. For this purpose, the transformer has a first winding which can be connected in parallel with the entire cell arrangement, and several second windings, each of which is connectable in parallel with one of the battery cells.
According to the invention, the transformer is expanded by an additional winding, which in the following will be called a third winding. The third winding is provided for feeding energy from an external energy source. As an example, the external energy source may be a solar cell arrangement.
Such a battery system would be suitable for use in a vehicle, particularly a hybrid vehicle or in a purely electric vehicle. It is contemplated to provide the solar cell arrangement on an exterior side of a vehicle body component, for example, on a vehicle roof and to use it for charging the battery system.
According to a further aspect of the invention, the battery system has a “high-voltage cell arrangement” with an operating voltage of more than 300 volt. Such systems are particularly suitable for applications in hybrid or electric vehicles.
It is, in principle, also possible to feed the electric energy supplied by a solar cell arrangement directly into the cell arrangement or into individual battery cells and not by way of a transformer as described herein. However, in the case of high-voltage batteries, since they are used in hybrid or electric vehicles, this is not acceptable for safety reasons. In the case of vehicles with high-voltage batteries, for safety reasons, a galvanic separation is required between a “high-voltage area” and a “low voltage area” of the electric system.
A further advantage of the invention is the fact that a transformer arrangement provided for an active charge balancing can be expanded at comparably low expenditures by a “third winding”, by way of which energy can be fed from an external energy source.
As an alternative to a solar cell arrangement, electric energy could also be fed into the battery system from another energy source. Peltier elements, for example, could be considered, which utilize the thermal energy contained in the exhaust gas of an internal-combustion engine for generating electric current and feeding the latter by way of the third winding into the battery system.
Research has shown that, by means of currently available solar cells and a solar surface of the size of a conventional car roof, under solar radiation conditions, as they occur in Central Europe for example, energy of approximately 600 wh a day can be generated, whereby at least several starting operations of an internal-combustion engine can be carried out, or an average vehicle can be accelerated several times from a stationary position to a speed of approximately 50 km/h.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.